The present invention generally relates to an Auxiliary Power Unit (APU) system of the type utilized for starting a jet engine and providing environmental control for commercial aircraft. More particularly, the present invention is directed to an inlet duct and interfacing plenum capable of significantly reducing Exhaust Gas Temperature (EGT) which directly translates to reducing Turbine Inlet Temperature (TIT) and, consequently, increasing APU turbine life.
It has been observed in prior systems that plenums that receive uniform airflow tend to produce a pair of vortices that cause a 2 per rev mechanical and aerodynamic distortion at the compressor eye 10 as shown in FIG. 1. For each revolution of the compressor 12, a blade leading edge 14 experiences a reversal of flow incidence two separate times. This distortion causes a decrement in compressor performance that leads to increased TIT and ultimately reduced turbine life as well as reduced operating efficiency of the APU.
Compressors prefer a reasonable amount of counter-rotating bulk swirl. Any non-uniform flow entering a plenum generates bulk swirl. Air having a controlled velocity profile entering the plenum can be used to create such a favorable counter-rotating bulk swirl. It has been further observed that a simple straight-walled inlet duct that turns substantially 90xc2x0 into a plenum tends to produce a uniform plenum flow resulting in the reduced life of the turbine blades and ultimately the APU system.
There is a need for a bulk swirl generator having an inlet into a plenum that generates air flow having a predictable and prescribed velocity gradient entering the plenum and that assures the desirable bulk swirl is maintained within an APU plenum to reduce EGT and ultimately TIT in order to maximize life of the turbine blades.
In one aspect of the present invention, a bulk swirl generator comprises an airflow passageway leading to an APU plenum, which feeds to a compressor. The passageway creates a non-uniform air flow profile or gradient entering the compressor. In particular, a tapered ramp extends across the width of an inlet duct at the interface of the duct and the APU plenum. The ramp has a maximum turn radius located at one side of the inlet duct, steadily decreasing in radius or taper until a sharp corner is formed at the opposite side of the inlet duct. The turn angle of the radius is preferably between about 45xc2x0-90xc2x0. The tapered ramp creates the non-uniform velocity gradient across the mouth of the compressor plenum with the greater velocity existing at the side of the plenum inlet duct having the greatest turn radius. The intersecting surfaces of the inlet duct and the plenum oppositely disposed from the ramp form an angle of substantially 90xc2x0. The velocity of the stream of air entering the plenum steadily decreases as the radius of the ramp decreases, creating a non-uniform velocity gradient entering the plenum. This, in turn, creates the desired bulk swirl in the opposite direction to the rotation of the compressor rotor.
In another aspect of the invention, an air flow passageway comprises a compressor plenum and attached inlet duct wherein a fillet is created at the interface between the inlet duct and the mouth of the plenum. The fillet tapers from a maximum radius at one side of the inlet duct to a sharp corner at the other side of the inlet duct. The opposite interface between the inlet duct and plenum forms an angle of approximately 90xc2x0. The tapering fillet creates a non-uniform velocity gradient entering the plenum that, in turn, creates the desirable bulk swirl flow in the direction opposite to the rotation of the compressor rotor.
In still another aspect of the invention, a flow passageway generator includes a rounded interface between the inlet duct and the compressor plenum. The rounded interface uniformly tapers from having a maximum radius at one side of the inlet duct to having a sharp corner at the other side of the inlet duct. The inlet duct and plenum interface oppositely disposed from the rounded interface form an angle of substantially 90xc2x0. This configuration creates a flow passageway having a non-uniform gradient wherein the maximum airflow velocity is created adjacent the portion of the rounded interface having the maximum radius; with the air flow velocity decreasing toward the opposite side with the sharp radius or corner.
In a further aspect of the present invention, a generator comprises an air flow passageway wherein a ramp is disposed in the wall of a compressor plenum, with the ramp extending upstream towards the interface with an inlet duct attached to the plenum. The leading edge of the ramp extends across the mouth of the plenum and is formed with a maximum turn radius located at one side of the plenum. The turn radius steadily decreases in radius or taper until reaching a sharp corner at the opposite side of the plenum. The interface inlet duct and plenum interface opposite the ramp forms an angle of substantially 90xc2x0. The radius of the ramp turn angle is preferably between about 45xc2x0-90xc2x0. During operation, air flows through the inlet duct and across the plenum ramp toward the compressor rotor. The tapered ramp creates a non-uniform velocity gradient that, in turn, creates the desirable bulk swirl in the opposite direction to the rotation of the compressor rotor. The bulk swirl reduces the EGT and ultimately the TIT, significantly increasing the life of the APU turbine blades as well as improving the operating efficiency of the APU by decreasing the amount of fuel needed to run the system.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.